Production of acrylate-terminated urethane polybutadienes has been described in the patent literature since the end of the 1960s. Japan Soda describes the preparation of these types of functionalized polybutadienes in numerous patents, such as DE1944015. Similar products have also been mentioned in U.S. Pat. No. 3,855,379, DE2244918A1, JP49117588A, JP49117595A, JP50153088A, JP50153091A, JP50150792A, GB1575584A, DE2702708A1, DE2737174A1, DE2821500A1, JP56060441A, BR8101674A, JP59021544A, CA1253289A1, JP60195150A, JP60151260A, JP61006155A, JP61021120A, JP60195038A, and JP61123649A. The use of tin catalysts was published in JP2002371101A, while patents EP2910578A1 and U.S. Pat. No. 8,822,600B2 describe the use of organoaluminum, organozinc, and, by analogy, organobismuth and organozirconium as catalysts. With catalysts such as these, viscosity does not rise much as a function of time, giving them an advantage over catalysts that contain Sn. New acrylate-terminated urethane polybutadienes with previously unknown properties remain in demand today.
Acrylate-terminated urethane polybutadienes are used as adhesion promoters in optical devices (JP2015147916A, WO2014196415), in OCRs (optically clear resins) (JP2015071682A), in sealing compounds (JP2010265402A), in insulating coatings (JP2008291114A), as pressure-sensitive adhesives (PSAs) (US 20110076491 A1, JP2003155455), in optical fibers (19, JP06021138B), resins for pressure plates (JP010283658 and DE2702708B2), etc. There is very little in the literature, however, on the use of acrylate-terminated urethane polybutadienes in optical applications for electronics—especially in LOCAs.
Optically clear adhesives and especially liquid optically clear adhesives are finding wide applications in optical displays. Optical bonding in display applications is used to bond optical elements such as display panels, glass plates, touch panels and flexible films such as polarizers and retarders, especially the use of such adhesives for bonding in touch displays, for example, capacitive touch display is high interest. The importance of optically clear adhesives is still growing since the continuous development of new electronic display products, such as wireless reading devices, increases the demands for optically clear adhesives.
For some applications, like OLEDs, it is further desirable to have an adhesive which shows low outgassing. For example, U.S. Pat. No. 8,253,329 mentions that small molecules that are outgassed from adhesives can potentially damage the OLED device when the adhesive is in direct contact with the OLED device. Hence, it is technically desirable to use for such adhesives materials, especially oligomers, with a low amount of low molecular weight side products and hence lower outgassing.
WO 2014/196415 A1 describes a photocurable resin and a photocurable resin composition. The therein described formulations have a monomer content of 50% per weight that will not allow to make a formulation that is able to be used to realize cured films with low modulus and high flexibility. In addition, the use of high monomer contents in formulations very often lead to haze, yellowing and shrinkage issues that negatively affect the appearance of the films.
CN104231996 A discloses an optically transparent compound comprising a polybutadiene-polyether block copolymer in order to improve the yellowing, haze and shrinkage properties. However, this oligomer is reducing the overall adhesion of the formulation system. Hence it is technically desired to have a formulation that does not require the use of a polybutadiene polyether acrylate in order to reach low yellowing, haze and shrinkage.
In order to accelerate curing CN102703019 proposes to use thiol compounds in liquid optical transparent plastics. However, such thiol compounds are known to lead to issues in respect to high yellowing and therefore have a negative effect on the optical properties.
JP2003155455 describes formulations for pressure sensitive adhesives (PSA). For such PSAs it is common practice to use solvents to lower the viscosity of the composition and to make the material processable. If the formulation disclosed by this citation is used in a liquid optically clear adhesive (LOCA), it will lead to high yellowing and low flexibility. In order to improve these two properties it is necessary to add a liquid rubber, which will lead to issues regarding compatibility and haze.
There are no descriptions in the literature on the use of acrylate-terminated urethane polybutadienes from low-monomer 1:1 monoadducts from reactive olefinic compounds and diisocyanates and hydroxy-terminated polybutadienes for liquid optically clear adhesives (LOCAs).
For many optical applications—and especially for LOCAs—low formulation viscosity is key, as it makes the materials easier to apply. It is the objective of the present invention to provide formulations based on acrylate-terminated urethane polybutadienes with lower solution viscosity than that of prior art formulations.